1. Field of the Invention
The present invention relates to insulated wires used for coils in electrical equipment such as rotary electric machines and transformers. More particularly, the invention relates to insulated wires covered with at least an extrusion coated insulation layer.
2. Description of Related Art
Insulated or enameled wires are used for coils in electrical equipment such as rotary electric machines and transformers. Such insulated wires are typically formed by applying one or more insulation coatings around a metal conductor having a desired cross section (such as circular and rectangular) depending on the shape and application of the coil. Typically, insulation coatings are formed by the following two methods: One method is to apply, on a wire conductor, an insulation varnish prepared by dissolving a resin in an organic solvent and baking the applied varnish. The other method is to extrusion coat a preblended resin composition on a wire conductor.
Because of the recent demand for compact electrical equipment, insulated wires are wound around a smaller diameter core with a finer pitch under a higher tension in current coil winding processes. Insulation coatings for such insulated wires require sufficient mechanical properties (such as adhesiveness and wear resistance) to withstand severe mechanical stresses caused by such harsh coil winding processes.
Also, because of the recent demand for high efficiency and high output power electrical equipment, there has been an increasing use of inverters and high voltages. As a result, coils are subjected to higher operating temperatures. Hence, insulation coatings also require high thermal resistance. In addition, high voltages (such as surge voltages from an inverter) applied to a coil may generate partial discharges, thus potentially degrading or damaging the insulation coating.
In order to prevent degradation or damage of insulation coatings by partial discharge, insulation coatings having a higher partial discharge inception voltage are being actively developed. One exemplary method for increasing the partial discharge inception voltage of an insulation coating is to use a low dielectric constant resin for the insulation coating. Another exemplary method is to thicken the insulation coating.
For example, JP-A 2002-56720 discloses an insulation coating material containing a fluorine-containing polyimide resin having a special structure. The relative dielectric constant of the insulation coating material of this disclosure is 2.3 to 2.8, which is significantly lower than those of conventional insulation varnishes (about 3 to 4). According to this disclosure, heat generation in the insulation coating can be suppressed because of the low dielectric constant of the coating material.
JP-B 4177295 discloses an insulated wire which is resistant to voltage surges from an inverter. This insulated wire includes: at least one enamel layer that is applied on a wire conductor (or an underlying enamel layer) and baked; and at least one extrusion coated resin layer on the at least one enamel layer. The total thickness of the insulation coating is 60 μm or more, and the total thickness of the at least one enamel layer is 50 μm or less. Each extrusion coated resin layer is made of a resin that, except for polyether ether ketone, has a tensile modulus of elasticity of 1000 MPa or higher at 25° C., and 10 MPa or higher at 250° C. According to this disclosure, the insulated wire has a high partial discharge inception voltage (about 900 V) while maintaining a strong adhesion between the wire conductor and the insulation coating.
WO2005/106898 discloses an insulated wire formed by extrusion coating, on a wire conductor, with two or more insulation layers. At least one of the insulation layers other than the innermost layer is made of a resin mixture including 100 parts by mass of a polyphenylene sulfide resin as a continuous phase and 3 to 40 parts by mass of an olefin-based copolymer as a dispersed phase. According to this disclosure, the insulated wire has excellent thermal and chemical resistance.
The above-cited technologies have the following problems or disadvantages: The above JP-A 2002-56720 technology can reduce the dielectric constant of an insulation coating by making the coating using the disclosed fluorine-containing polyimide resin. However, generally, insulation coatings made of a fluorine-containing polyimide resin have poor adhesion to wire conductors. Thus, an insulation coating made of the fluorine-containing polyimide resin of the JP-A 2002-56720 may be lifted off from a wire conductor by severe mechanical stresses caused by harsh processes such as winding, thereby potentially causing dielectric breakdown of the coating in the worst case scenario.
The above JP-B 4177295 technology increases the partial discharge inception voltage of the insulated wire by increasing the thickness of the extrusion coated resin layer. Furthermore, in order to increase the adhesion between the wire conductor and the extrusion coated resin layer, a baked enamel layer is interposed therebetween, and in a preferred embodiment, an adhesive layer is further interposed between the enamel layer and the extrusion coated resin layer.
However, the properties and method of formation of the enamel layer are significantly different from those of the extrusion coated resin layer, thus adding to the complexity of the manufacturing process of the insulated wire and as a result leading to an increase in the manufacturing cost. When the adhesive layer is used, the manufacturing cost further increases.